The mechanistic target of rapamycin complex 1 (mTORC1) protein kinase is a master growth regulator. It integrates inputs from growth factors as well as nutrient and energy supplies to control key biosynthetic and catabolic processes, including protein translation, ribosome biogenesis and autophagy. As might be expected, this pathway is deregulated in a number of human diseases, ranging from neurodegeneration to cancer. Amongst the diverse inputs that impinge on mTORC1, amino acids are unique in that they are absolutely required for kinase activity, despite the presence of multiple activating stimuli. Amino acids utilize an independent mechanism from other cues to regulate mTORC1; however, this mechanism remains poorly characterized despite recent progress. By gaining a more complete understanding of how amino acids signal to mTORC1, we may also gain insight into more effective targets for pharmacological inhibition of mTORC1 with minimized side effects. We have used an unbiased proteomic approach to identify a novel octameric complex consisting of two interacting subcomplexes. The first subcomplex, MWS, is a positive regulator of the amino acid branch and contains Mios, WDR24, WDR59, Sec13, and Seh1L. The second subcomplex, DEN, contains DEPDC5, Nprl2, and Nprl3 and is the first negative regulator of the amino acid branch of mTORC1 to be identified. DEN's inhibitory role stems from its function as a GTPase activating protein (GAP) for the RagA/B GTPases, a key activator of the mTORC1 pathway. Importantly, as a negative regulator, DEN has the potential to be deregulated in cancer as a tumor suppressor. The goal of this project is to define and characterize the mechanisms of DEN and MWS in amino acid signaling to mTORC1 and to interrogate the possible role of DEN in carcinogenesis. To achieve this goal, we propose the following aims: 1. Further characterize the GAP activity of DEN towards RagA/B. 2. Determine the mechanism by which MWS regulates DEN in response to amino acids by exploring localization and a potential ubiquitin ligase function for MWS. 3. Interrogate the role for DEN as a tumor suppressor complex in cancer. Through a combination of hypothesis driven and unbiased approaches, our work will clarify how the amino acid pathway of mTORC1 functions in normal cells and how it can be deregulated in carcinogenesis. These insights may lead to the development of novel biomarkers and therapeutic strategies for cancer.